1. Field of the Invention
The present invention relates to a mobile communication control method and system and a mobile station, and more particularly to a system for determining a base station for transmission during hand-over in a cellular communication system.
2. Description of the Prior Art
In a cellular system employing a direct sequence code division multiple access, since a same frequency band is used by a plurality of channels, transmission for other channels makes interference, in which if interference is increased, the received signal quality of desired signal is degraded, causing a disconnection of a link. Accordingly, the trafic capacity, that is, the number of links capable of making communications while retaining a certain speech quality, depends on the amount of interference.
Therefore, in a downlink of the cellular system employing the direct sequence code division multiple access, a transmission power value of the base station is controlled to be the minimum transmission power value so that the received signal quality at the mobile station can retain a reference level. This involves a closed loop control of measuring the received signal quality at the mobile station, and transmitting a signal for instructing to decrease the transmission power value if it is above a predetermined target value, or transmitting a signal for instructing to increase the transmission power value if it is below the predetermined target value.
Typically, in the cellular system employing the direct sequence code division multiple access, a soft handover is employed. With this soft handover, when the mobile station comes near the cell boundary of a connecting base station, with a difference in propagation loss between the connecting base station and its adjacent base station below a certain value, the link is set with the adjacent base station to make connection at the same time. Thereby, a site diversity effect for allowing the transmission from a plurality of base stations can be obtained even in an area with a large propagation loss near the cell boundary, whereby the received signal quality is prevented from degrading, and the uninterrupted handover can be realized by setting in advance the link with a next base station to connect.
However, since in the soft handover a plurality of base stations are transmitted to one mobile station, there is a problem with the soft handover that interference increases in the downlink, and the trafic capacity decreases. As a technique to solve such a problem, a site selection diversity transmit power control system was disclosed in Japanese Patent Laid-Open No. 11-69416 in which the trafic capacity in the downlink was increased by designating a base station for actually performing the transmission among a group of base stations during the soft handover.
In this site selection diversity transmit power control system, the mobile station determines the base station with the least propagation loss and the base stations of which differences in propagation losses between the least propagation loss are below a predetermined threshold value, among the handover base stations, as the transmitting base stations. Them, the mobile station notifies the transmitting base station to the handover base stations. In the handover base stations, when the base station is not designated as the transmitting base station in accordance with a notification from the mobile station, that is, the non-transmitting base station, the transmission of the base station is stopped, whereby the interference on the downlink is suppressed.
Moreover, in the site selection diversity transmit power as described in “3G TS 25.214 v3.2.0 (2000–03) 3rd Generation Partnership Project: Technical Specification Group Radio Access Network; Physical layer procedures (FDD) pp.19 5.2.1.4”, among a dedicated control signal and a dedicated data signal which the base stations under communication transmit to the mobile station, it is only the dedicated data signal that the non-transmitting base station stops the transmission, and the dedicated control signal is transmitted at a power value under the normal transmission power control of fast closed loop type. Also, the power value of the dedicated data signal according to the transmission power control is always retained even at the non-transmission base station.
With these measures, synchronization is assured at the non-transmission base station, and the dedicated data signal can be transmitted at a power value according to the transmission power control of fast closed loop type immediately after being switched to the transmitting base station, whereby the communication quality in switching can be enhanced.
However, in the transmission power control of fast closed loop type, each handover base station receives a signal for instructing to increase or decrease the transmission power value from the mobile station and makes the independent control, whereby in some cases each handover base station has a different transmission power value due to a signal reception error.
Furthermore, the transmission power control of fast closed loop type, which is made so that the dedicated data signal at the mobile station has a predetermined received signal quality, has no effect on determining the signal for instructing to increase or decrease the transmission power value at the mobile station even in the case where the non-transmitting base station transmits the dedicated control signal at an excessive transmission power value, whereby the non-transmitting base station continues the transmission at the excessive transmission power value. This will be set forth in detail by reference to FIGS. 9 and 10.
FIGS. 9 and 10 represent the transmission power values of the handover base stations BS1 and BS2, and the total transmission power value of two base stations with the elapse of time. FIG. 9 involves an instance of the soft handover under the normal transmission power control, and FIG. 10 involves an instance of the handover under the site selection diversity transmit power control system. In each of FIGS. 9 and 10, the solid line represents the dedicated data signal, and the dotted line represents the dedicated control signal.
The mobile station transmits a transmission power control signal to the handover base stations, to instruct to decrease the transmission power value when a receiving SIR (signal to interference power ratio) of the dedicated data signal to be received is higher than the target SIR, or instruct to increase the transmission power value when it is lower than the target SIR, whereby each base station increases or decreases the transmission power value for the dedicated data signal and the dedicated control signal in accordance with the transmission power control signal. The transmission power control signal is received by each base station to control the transmission power value independently, whereby there is a difference in the transmission power value between handover base stations due to a reception error at the base station.
FIGS. 9 and 10 shows an instance where a reception error occurs at the base station BS2 during T1–T2 and generates a difference in the transmission power values. In a case of the normal soft handover as shown in FIG. 9, if the transmission power value of the base station BS2 is higher, the total transmission power value of the dedicated data signal for the mobile station is increased, so that the reception SIR at the mobile station exceeds the target SIR, and the base station BS2 is instructed to decrease the transmission power value using the transmission power control signal. Therefore, the base stations BS1 and BS2 decrease the transmission power value, thereby reducing the total transmission power value in transmission.
On one hand, in a case of the handover in the site selection diversity transmit power control system as shown in FIG. 10, a transmission base station BS1 transmits both the dedicated control signal and the dedicated data signal, while a non-transmission base station BS2 transmits only the dedicated control signal, and does not transmit the dedicated data signal. Hence, the base station BS2 produces a reception error during T1–T2, so that the total transmission power value of the dedicated data signal to the mobile station is not increased even if the transmission power value is higher. Accordingly, the reception SIR of the dedicated data signal at the mobile station is not increased, and no transmission power control signal is sent for instructing to decrease the higher transmission power value due to a reception error at the base station BS2. Therefore, the transmission power of the dedicated control signal at the non-transmission base station remains high.
In this state, though the non-transmitting base station stops the transmission of dedicated data signal to decrease the transmission power value, the total transmission power value transmitted by the base stations during the handover is increased owing to an increased transmission power value of the dedicated control signal. Therefore, the interference on the other mobile stations increases and the trafic capacity decreases. In particular, in the case where the transmission rate is slower, the transmission power distribution of the dedicated control signal is greater, causing a more significant influence.
With the site selection diversity transmit power control, the number of base stations for transmission is smaller than in the normal soft handover, bringing about a problem that if there is a significant propagation loss, the received signal quality may be insufficient and degraded even if the transmission power value is increased to the maximum transmission power value of the base station.